i18n: advice: internationalize message for conflicts

i18n: advice: internationalize message for conflicts

Mark message for translation telling the user she has conflicts to
resolve. Expose each particular use case, in order to enable translating
entire sentences which would facilitate translating into other
languages.

Change "Pull" to lowercase to match other instances. Update test
t5520-pull.sh, that relied on the old error message, to use the new one.

Although we loose in source code conciseness, we would gain better
translations because translators can 1) translate the entire sentence,
including those terms concerning Git (committing, merging, etc) 2) have
leeway to adapt to their languages.

Signed-off-by: Vasco Almeida <vascomalmeida@sapo.pt>
Signed-off-by: Junio C Hamano <gitster@pobox.com>